Update pom.xml to point correct sourceDirectory

No need to specify sourceDirectory if the project follows maven
standard. With the fix, I removed unnecessary definitions for
maven-compiler-plugin.

I guess this wrong definitions are from my temporary pom.xml definition.
Sorry for confusing!
configurations for sbt.
Merge branch 'dev'
Merge branch 'incorporate_netty_fixes'
Merge branch 'dev'
Merge branch 'dev'
Merge branch 'dev'
improving error diagnosis.

This is done to better understand reported problems such as https://issues.jenkins-ci.org/browse/JENKINS-20618 and https://issues.jenkins-ci.org/browse/JENKINS-20114 where jzlib can be seen mysteriously failing.
Merge pull request #11 from kohsuke/master

improving error diagnosis.